In recent years, improvements in the speed and capacity of optical communication have been accelerating, and optical communication systems with an extra-large capacity of 40 Gbps have been introduced. Moreover, research and development toward utilization of next-generation 100 Gbps optical communication systems have also been actively performed. A number of inductors (coils) are used as application of a bias T in a high-frequency circuit of an optical transmitter/receiver or a measurement instrument adopted as these optical communication systems, and the demand for coils that are excellent in high frequency characteristics is increasingly growing.
In the bias T to be used in about 10 Gbps optical communication, a small surface-mount-type coil (for example, surface-mount-type coil with a size of about 1.0 mm×0.5 mm) has been used until now. Even if such a coil is used, conspicuous degradation of high-frequency characteristics up to about 10 GHz has not been seen. However, since high-frequency characteristics that are satisfactory up to about 40 GHz are required in a high-frequency circuit to be used for optical communication of 40 Gbps or more, the above surface-mount-type coil cannot be used.
Thus, in order to obtain high impedance in a broad frequency band, a winding type coil with a configuration in which the diameter of the coil changes continuously is proposed (for example, refer to Patent Document 1). Since this constitutes a plurality of different inductors by one coil by gradually increasing the diameter of the coil, the function as an inductor can be favorably exhibited in a very broad frequency band.
However, such a winding type coil has problems in that mounting onto a circuit substrate is difficult, and handling is also not easy. Moreover, such a winding type coil also has problems in that variation of characteristics during mounting, such as occurrence of a difference in characteristics caused by a difference in mounting angle, is large.
Thus, a coil (for example, refer to Patent Document 2) that solves such a problem is also proposed. The coil disclosed in Patent Document 2, which is formed on a substrate including a plurality of layers, includes a transmission line mounted on this substrate, and transmission line patterns that generates inductors, and has a structure in which the transmission line patterns are electrically connected to each other through vias connecting the layers of the substrate so as to become three-dimensional conical inductors.